The creation of well-defined micron-scale features upon a surface is the foundation of the microelectronics industry. Small features raise the efficiency of a microelectronic device and, ultimately, the ability to create smaller features drives advancement in the semiconductor industry. Photolithography has been the dominant method for fabrication of microcircuitry since the first semiconductor transistor circuit was developed. In a photolithography process, a monochromatic light is projected through a mask to create, generally after a development step, a pattern in a resist material supported on a substrate, generally a semiconductor. The patterned resist material protects portions of the substrate during a subsequent doping or etching processes of the exposed areas to alter the electronic or topographic features of the substrate.
Photolithography relies upon focusing of a shadow pattern produced from a mask to reduce feature sizes. When a “perfect” lens is used, the resolution of the features is said to be diffraction limited, where the minimum feature size attainable is limited only by the size of features in the mask where diffraction of the light projected through it occurs. Hence, improvement of the resolution of features attainable with photolithography requires a smaller wavelength of light so that a smaller aperture can be used.
Of the five identified alternatives to optical lithography for sub 50 nm lithography: extreme ultraviolet (EUV); x-ray; imprint; ion beam projection; and electron beam lithography (EBL), EBL is the most mature. EBL provides a solution to the problem of diffraction limitation.
The wavelength of an electron accelerated by a potential is given by:λ=1.2/(Vb)1/2 where Vb is the accelerating voltage of the electron beam. For example, at 25 kV the wavelength of the electron is 0.008 nm. For direct-write applications, the electron beam can be delivered by a scanning electron microscope where resolution is not fixed by the diffraction limit, but rather resolution is limited primarily by the diameter to which the electron beam can be focused. Electron beams have been focused to less than 1 nm, and the ultimate practical limit for the resolution of electron beam lithography is considered to be about 7 nm.
As with all resists, electron beam resists can be divided into two broad classes: positive tone resists that are removed more easily after exposure to the radiation and negative tone resists that are made more resistant to removal after exposure to the radiation. A typical mechanism for forming a negative tone resist is an increase in molecular weight by polymerization or cross-linking to cause a decrease in solubility of the exposed area to a developer. A positive tone resist mechanism typically involves a lowering of molecular weight through chain scission or involves a functional group conversion to cause an increase in the solubility of the exposed area to a developer.
The sensitivity of the resist material is of fundamental importance. The sensitivity of an electron beam resist relates to the dose of electrons required to expose an area of the film. The more sensitive the resist is to the electron beam, the faster it can be processed. This sensitivity is typically determined by exposing the resist to a range of electron doses, where after development, the thickness of the film in the exposed areas is measured. In the case of a negative tone electron beam resist, the thickness increases with dose.
The best commercially available electron beam resists are relatively simple polymers, such as poly(methyl methacrylate) (PMMA). PMMA was one of the first electron beam resists, discovered shortly after the application of SEM to lithography and is commonly used as a standard for comparison with other electron beam resists. PMMA is commonly supplied for electron beam resists as 496,000 or 950,000 g/mole molecular weight (MW) polymer as a chlorobenzene or anisole solution. It is then typically deposited by spin coating followed by baking at 170-200° C. to form the resist film on the substrate. PMMA can act as either a positive or a negative tone electron beam resist, depending on the dose of electrons delivered to it.
The sensitivity of PMMA in the positive tone dose regime depends on the electron beam accelerating voltage. The sensitivity decreases from 1.0×10−5 C cm−2 at 2 keV, to 9×10−5 C cm−2 at 20 keV, and to 3.5×10−4 C cm−2 at 50 keV. The resolution of positive tone PMMA is very high under optimal lithographic conditions with, for example, 6 nm features formed using 1,100,000 MW PMMA at 80 keV followed by development with a 3:7 cellosolve:methanol solution.
In the negative tone dose regime, the sensitivity of PMMA is lower (˜1×10−3 C cm−2) with 10-20 nm features formed. However, because the etch durability of PMMA is lower than that of a silicon, PMMA's utility has some limitations. This shortcoming has been addressed by the addition of C60 or a C60 derivative to the PMMA, as disclosed in Ishii et al., U.S. Pat. No. 6,177,231. The fullerene particles increase the etch durability of the PMMA with an enhancement of the contrast of the resist pattern upon development with a solution appropriate for the fullerene particles. Similar improvements to positive tone resists are also achieved. The fullerene particles were used to enhance the etching resistance by reducing free space within the resist film and inhibiting penetration of etching reactive species into the resist film. The sensitivity of the PMMA containing fullerene particles was an order of magnitude higher than fullerene free PMMA, and superior lines are formed using doses of about 5 μC cm−2.
Tada et al., Jpn. J. Appl. Phys., 1996, Part 2, 35, L63, discloses C60 as a negative tone electron beam resist material. Si patterns with feature sizes in silicon as low as 20 nm were achieved. The etch durability of C60 is excellent, being 10/1 compared with silicon. C60, however, is insoluble in water and only sparingly soluble in organic solvents, making it unsuitable for spin coating, and its sensitivity is much lower (1×10−2 C cm−2) than PMMA (5.0×10−5 C cm−2).
Tada et al., J. Photopolym. Sci. Technol., 2001, 14, 4, 543, discloses various modified C60 derivatives that dramatically improve the film forming and electron beam resist properties relative to C60. The C60 derivatives were spin coated as a 300 nm thick film, exposed to a 20 keV electron beam, and developed using chlorobenzene. It was shown that these negative tone resists display better sensitivity (2.5×10−3 to 3.8×10−4 C cm−2) than that of C60 but a lower sensitivity than PMMA.
Hence, although an improvement of the electron beam resist's etch resistance by the inclusion of fullerene particles has been demonstrated, other limitations still exist in the process that, if eliminated or improved, could improve the contrast and resolution of the process. For example, the development step of the exposed film relies on the solubility differences of the exposed and unexposed resist. Improving this development step has the potential to significantly improve the overall lithographic process. Therefore, a lithographic process that directly forms the pattern without development is highly desirable.